1. Field of the Invention
The present invention generally relates to image data processing and, more particularly, to a digital image processing technique which improves an image quality by reducing moiré fringes generated in a fine-pitch halftone image.
2. Description of the Related Art
Conventionally, in a digital copy machine, an unevenness of intensity referred to as moiré is generated in a reproduced image of a halftone image, which deteriorates an image quality. There are following causes of generation of moiré.
1) a mutual interference between a frequency of dots in an original image and a sampling frequency for resolving the original image into pixels when reading the original image; and
2) a saturation operation by spatial filtering means, or a nonlinear process such as sharp gamma-correction process.
As for a method of suppressing generation of moiré due to the above-mentioned cause 1), there is a method of reducing moiré by a smoothing filter process, which makes an input image unfocused. As for a method of suppressing generation of moiré due to the above-mentioned cause 2), there is a method of reducing moiré by applying a weak filtering process, which cannot cause saturation, or a smooth gamma correction process.
Japanese Laid-Open Patent Application No. 5-41793 discloses a digital image processing apparatus which corrects moiré prior to a spatial filtering process including a modulation transfer function (MTF) correction such as sharpening of character edges and smoothing of a halftone image. In this example of a moiré correction, input image data of 400 dpi (dot/inch) is processed by an interpolation method so as to compute interpolated image data corresponding a position between adjacent pixels in the input image data, and a smoothing process is applied to an assemble (image data of 800 dpi) of the input image data and the interpolated image data so as to produce image data of 400 dpi.
Japanese Laid-Open Patent Application No. 9-9038 discloses an image forming apparatus, which performs a laser recording operation based on binary data obtained by binarizing image data. In this image forming apparatus, a spatial filtering process is applied to the image data obtained by a sampling frequency of 400 dpi so as to correct sharpness and remove moiré. Then a gamma processing which is an intensity correction is applied to the image data, and the image data is subjected to a high-density resolution conversion process to 600 dpi. Thereafter, the binarizing process is applied to the 600 dpi image data.
Additionally, Japanese Laid-Open Patent Application No. 10-32713 also discloses an image forming apparatus, which performs a laser recording operation based on binary data obtained by binarizing image data. In this image forming apparatus, the high-density resolution conversion process is applied first to convert image data from 400 dpi to 600 dpi. Then a spatial filtering process and gamma-correction process are applied to the image data of 600 dpi, and the processed image data is binarized to obtain the binary image data.
As mentioned above, the image processing operation using the above-mentioned high-resolution conversion process can reduce moiré generated in a reproduced image. However, the reproduced image may be unfocused, or resolution or sharpness may deteriorate in a line-drawing part such as characters. An image having less moiré may be obtained by increasing the image density for image reading. However, in order to increase the image density, a CCD reading apparatus having a corresponding pixel resolution capability and it is difficult to achieve such a CCD reading apparatus in respect of a technical matter and a manufacturing cost.